Internal combustion engines and static hydrocarbon burning equipment tend to emit, via their exhaust systems, carbonaceous particles commonly referred to as particulates. While efforts are being expended towards reducing particulate emissions at source, particulate filters (traps) in the exhaust systems of such equipment are becoming useful in helping to meet increasingly strict environmental legislation and public expectations.
Particulate filters which may be regenerated are known. It is especially desirable for a particulate filter to be self-regenerating in use, under any load, in order to maintain filtering and gas-flow efficiencies above a certain level while keeping filter size to a minimum. It is also desirable that the filter is self-controlled to regenerate when a predetermined level of particulates is present and to do so without requiring any external sensing means. It is further desirable that the regeneration process is economic in the use of any externally supplied energy or material, that the construction of the filter is also economic, and that the system is effective irrespective of types and compositions of fuel and engine operating conditions.
WO 01/04467 (“WO '467”) to the same applicant discloses an apparatus and a method for removing particulates from a gas stream. WO '467 discloses a ceramic monolith filter of depth less than 100 mm which uses a first electrode to produce an atmospheric glow discharge near to the first end of the filter. Although the combination of a reduced-depth ceramic filter and atmospheric glow discharge has provided far more efficient particulate removal and filter regeneration performance compared to earlier known arrangements, the applicant has established that performance can be improved still further.